A crystalline mixture of sterols was isolated from the filamentous blue-green alga Phormidium luridum. The mixture consisted of unsaturated 24-ethylcholesterols possessing Delta(7)-, Delta(5,7)-, and Delta(5)-bonds together with their Delta(22)-derivatives and a small amount of cholesterol. The major component was 24-ethyl-Delta(7)-cholestenol. Squalene and phytol were also evident.